1. Field of the Invention
This invention relates to a method of and an apparatus for making a hologram.
2. Description of the Prior Art
A hologram lens can be obtained by making the hologram of a point source of light by the use of the holography technique. The hologram lens has the advantage that it is a thin film lens of planar shape having a thickness of several microns and can be mass-produced on the same planar plate by the step-and-repeat method. Therefore, it has been proposed to utilize the hologram lens as the optical element in an optical system utilizing a laser light, such as the condensing lens of the optical head of an optical disc device or a collimation lens for converting the divergent light beam from a semiconductor laser into a parallel light beam.
In the optical system of the optical head unit of the optical disc device, a light-condensing hologram lens is disposed on the surface of a disc substrate so as to read signals recorded on the back of a plastic plate having a thickness of about 1.1 mm. The hologram lens is disposed with an air space of the order of 1 mm with respect to the disc substrate so that it may not collide against the disc substrate due to vibration of the disc, and a cover glass or protective layer of a suitable thickness is interposed therebetween to prevent adherence of dust or the like to the hologram lens.
A hologram lens making optical system used in such an optical system is shown in FIG. 1 of the accompanying drawings. In FIG. 1, part of a monochromatic light emitted from a laser light source 1 is transmitted through a half-mirror 3, is reflected by a reflecting mirror 4, and is condensed into a pin-hole 16 by a microscope objective lens 15, and the light transmitted through the pin-hole 16 is transmitted through a collimation lens 17 and becomes a parallel light beam 18, which is transmitted through parallel flat plate 9 and enters a hologram sensitive material 11 applied to a hologram substrate 10. This is a reference light. On the other hand, the light beam reflected by the half-mirror 3 is reflected by a reflecting mirror 5 and is condensed into a pin-hole 8 by a microscope objective lens 7, and the light transmitted through the pin-hole 8 becomes a divergent light beam 12, which is transmitted through the parallel flat plate 9 and enters the hologram sensitive material 11. This is an object light. The object light beam 12 is made into a divergent light beam having spherical aberration by the parallel flat plate 9, and this light beam and the reference light beam form interference fringes at the position of the hologram sensitive material 11, and these interference fringes are recorded on the hologram sensitive material 11. By developing this, there is obtained a hologram lens.
Where the hologram lens thus made is used, a laser light of the same wavelength as that used during the making of the hologram lens is caused to enter the hologram 11 as a parallel light beam at the same angle as the parallel light beam 18 but in the opposite direction. The light diffracted by the hologram 11 becomes a convergent light beam having the spherical aberration imparted to the object light during the making and, after this light beam has been transmitted through the cover glass and the disc substrate, a light spot is created at a position corresponding to the pin-hole 8 during the making of the hologram.
Thus, by using a light of the same wavelength during the making and during the use, complete wave surface reproduction can be accomplished substantially without aberrations by the hologram lens.
Particularly, where a volume type phase hologram is made by using bichromate gelatine as the hologram sensitive material 11, the diffraction efficiency of the hologram can be enhanced to about 100% and the utilization efficiency of light is of an acceptable value.
Now, as the light source in an optical system using a hologram, it is preferable to use a compact, light-weight semiconductor laser which does not require any special modulator. The oscillation wavelength range of such a semiconductor laser is usually from the near infrared range to the infrared range (0.78 .mu.m or more). Accordingly, where the making of the hologram lens as described above and the image reproduction using it is effected by the use of such semiconductor laser, it is necessary that use be made of a hologram sensitive material having the effective sensitivity in 0.78 .mu.m or more. As a hologram sensitive material having sensitivity in this wavelength range, there is a silver salt sensitive material sensitized by infrared light. However, a hologram made by the use of such sensitive material is an absorption type hologram and therefore, it has a disadvantage that its diffraction efficiency is as low as several %. Also, the diffraction efficiency can be improved to some degree as by bleaching such hologram, but there is a limit to this.
Accordingly, to improve the diffraction efficiency, it is necessary to adopt a volume type phase hologram. Bichromate gelatine is typical as the sensitive material used for the making of such a hologram. However, this sensitive material is such that its effective sensitivity area is up to green light of maximum 0.55 .mu.m. Even if special coloring matter sensitization is applied thereto, it is merely possible to endow it with sensitivity up to red light of 0.6 .mu.m. Further, sensitive material for volume type holograms having an effective sensitivity in the near infrared range and the infrared range is not yet known.
Therefore, a semiconductor laser cannot be used during the making of a volume type phase hologram. Instead, use is made of a laser having a shorter wavelength. When a hologram thus made is used in an optical system using a semiconductor laser, the wavelength of light differs during the making and during the use. Therefore, imaging does not occur without aberrations and accordingly, in some cases, aberration correction becomes necessary. Where the making of a hologram is effected with aberrations being pre-imparted to the forming light beam with the aberrations resulting from the difference in wavelength between the radiation during its use being taken into account, it then is necessary to make a reference wave and an object wave by the use of independent optical systems and to dispose them in a predetermined space. Thus, the placement accuracy of the hologram making optical system is very critical. Where many holograms are to be made by the making optical system of which such critical placement accuracy is required, if the optical system deviates from its predetermined setting due to the vibration from the environment re-setting will be required each time such deviation occurs. Accordingly, it is not desirable to mass-produce holograms by the use of such an optical system.
The foregoing description has been made of a hologram lens for condensing a parallel light beam at a point. Another difficulty occurs for hologram lens for converting a divergent light beam into a convergent light beam. FIG. 2 of the accompanying drawings shows an optical system for making a hologram lens for condensing a light beam diverging from a point at another point. In FIG. 2, it is necessary that a reference light beam 18 converge at a point 20 in the air without aberrations after it has emerged from a hologram substrate 10. However, since this light beam 18 passes through a parallel flat plate 9 and hologram substrate 10, aberrations occur due to these optical elements to negate these aberrations, it is necessary to provide a single lens 17 and a cylindrical lens 17' obliquely with respect to the optic axis on this side of the parallel flat plate 9, for example. Actually, however, it is quite difficult to design such an optical system, make these optical elements and arrange them accurately. Accordingly, such an optical system is not suitable as the means for mass-producing hologram lenses.
Further, in the hologram making optical systems as shown in FIGS. 1 and 2, the parallel flat plate 9 is disposed immediately forward of the hologram sensitive material 11, and this results in creation of harmful ghost images. That is, light beam 13 which results from part of the object wave light beam 12 being reflected by the second surface and then by the first surface of the parallel flat plate 9 and a light beam 13' generated from part of the object wave light beam 12 being reflected by the surface of the hologram sensitive material 11 and thereafter the second surface of the parallel flat plate 9 enter the hologram sensitive material 11. Harmful ghost images are recorded. These ghost images are reproduced during the use of the hologram lens and may result in creation of unnecessary ghost light and reduction in diffraction efficiency.